Threats to soil, water put humanity's life supports in peril, biologist says

STANFORD - Current patterns of human activity imperil crucial elements of Earth's life support systems, a Stanford scientist told a meeting of the American Association for the Advancement of Science in Baltimore on Saturday, Feb. 10.

Speaking on "Biophysical and Social Aspects of Earth's Carrying Capacity," biologist Gretchen Daily reported recent findings on human use and abuse of two of Earth's most precious resources - fertile land and fresh water.

"These resources are absolutely crucial to human well-being," Daily said. There are no substitutes for either land or water in most of their uses, so their condition and availability represent important biophysical dimensions of the planet's capacity to support human activities.

Food production and general economic development are constrained where land and water are scarce. But Daily said that even in affluent nations, people are vulnerable to another long-range effect of scarcity: Pressures on soil and water supplies can promote the conditions that favor the spread of epidemic diseases.

Daily is Bing Interdisciplinary Research Scientist in Stanford's Department of Biological Sciences. She spoke at a session titled "International Perspectives on Population, Consumption and the Environment: Science and Policy Issues." She also was one of the speakers at a press briefing on consumption, population and the environment held on Feb. 9.

Soil depletion

More than 40 percent of Earth's vegetated land surface has been degraded over the last 50 years because of human activities, such as poor farming and grazing practices. "This is a big deal because productive land is a key economic asset - part of any nation's 'natural capital.' It takes hundreds to hundreds of thousands of years to generate a rich soil, an important component of productivity, but just a moment of carelessness in human history for it to be wasted away - forever, from society's perspective," Daily said.

In a study published in Science on July 21, 1995, Daily found that the planet has lost about 10 percent of its total productive value - that is, its capacity to provide crops and pasture, or to serve as a source of forestry, medicinal or industrial products. "Fortunately, Earth has an enormous potential for recovery, if humanity will foster it," she said at the AAAS session. "If concerted efforts are made to arrest land degradation and to encourage its recovery, the lost 10 percent could be reduced by half in 25 years."

On the other hand, if business continues as usual, Earth's lost productive value could increase to as much as 20 percent in that time period.

"Land mismanagement has been implicated in the fall of great civilizations. It is a problem worthy of much more serious attention," she noted.

Water use

Growing human demands have put similar pressures on Earth's fresh water supplies. How much renewable fresh water is there and how much does humanity have today? To answer these questions, Daily teamed up with Sandra Postel, director of the Global Water Policy Project, and with Stanford biologist Paul Ehrlich. Postel was the lead author of a report of their findings published in the Feb. 9 issue of Science.

To their surprise, the researchers found that irrigation, industry and municipalities already use more than half of all accessible runoff - the rainfall that falls on land and flows down to the sea. They then made optimistic projections of how much more runoff humans could gain access to, through construction of major dams. Taking into account projections of population size and water use per person, they found that water demand could grow to 70 percent - and possibly 100 percent - of total accessible runoff by 2025, a troubling scenario. Many aquatic systems already suffer severe ecological degradation, including decimation of fish populations and loss of recreational and many other values.

Humanity also uses about 26 percent of the global rainfall used by plants, because it falls onto croplands, timberlands, pastures and other managed systems. The prospects for expanding this are slim because most suitable lands already have been converted to these uses.

Daily said the most promising ways of slowing human demand for fresh water involve greater efficiency on the farm, both in irrigation technologies and in cropping patterns. In addition, pollution prevention could go a long way toward freeing up water supplies, since much of the water that humans appropriate from rivers, lakes and streams is used to dilute pollution.

"Land and water are sure to pose increasing constraints on food production and economic development generally," she concluded.

Land, water and disease

Daily also highlighted the intimate connections between patterns of land and water use and the growing threat of epidemic disease. "Everyone has a stake in the global land and water situation, whether living in developed or developing nations. The pathogens that are promoted through changes in the condition of Earth's lands and waters do not travel with passports - everyone is vulnerable."

Land clearing for agriculture, use of pesticides, and alteration of the hydrological cycle through dam construction all have enormous impacts on the "epidemiological environment." That environment consists of all the conditions that influence the interactions between human beings and disease agents, such as bacteria, viruses and malaria parasites.

"Development changes some of these conditions for the better, by improving water supplies and access to health care, for example," Daily remarked. But intensified human pressures on land and water also promote conditions that favor agents of disease.

As an example, she cited the enlarged populations of disease vectors, such as rodents and mosquitoes, that thrive in fields and flooded rice paddies. "We remove their predators and natural enemies, feed them and provide them with the perfect breeding environment in the course of trying to grow more food for ourselves," she said.

Clearing new agricultural land also brings human populations into contact with animal reservoirs of disease. Daily said, "The enormous human population represents the biggest jackpot in history for any pathogen capable of evolving host specificity to us - making a jump like HIV did."

Another cost of land clearance is the loss of biodiversity, the source of many modern pharmaceuticals as well as traditional medicines. Out of 150 top prescription drugs, 118 are based on chemical compounds from other organisms. Daily said that a conservative estimate of biodiversity loss is one species per hour, which exceeds by at least four orders of magnitude the rate of evolution of new species.

"As we try to improve our management of the biosphere to maintain all the good things in life for our children and grandchildren, it is essential that we consider carefully the tight linkages between different components of our life support systems and different parts of the globe," she said.

"We all depend on the health of this one unique planet in the universe - there is no life boat escape route for the rich."

-jb-

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